Bauingenieur 2022: Gentle pile breaking in confined spaces

Pile foundation

Pile foundations continue to gain in importance due to the increase in high-density construction and more frequent building in exposed locations. However, previous pile head processing methods involve risks that can be avoided with a newly developed method.

Building land is becoming increasingly scarce and the call for high-density construction is growing ever stronger. However, in exposed locations in particular, the available building ground increasingly has a poor load-bearing capacity. In such circumstances, pile foundations are the only solution to ensure a secure foundation. The pile head represents the connection to the foundation and must not fail. The piles are usually over-concreted during pile production for quality reasons (dirt inclusions) and the reinforcement must then be exposed again.

Conventional pile breaking

Until now, bored piles have mainly been processed with air or hydraulic hammers that work with vertical impact energy. The direction in which the energy spreads into the pile cannot be controlled. As a result, there is a risk of cracks in the concrete body and spalling on the outer skin of the pile (Fig. 1). This can cause corrosion damage in the medium term. Bent or torn reinforcements caused by air or hydraulic hammers further reduce the tensile load-bearing capacity.

Due to their mode of operation, air and hydraulic hammers also require a large working area due to their design. They make it difficult to process narrow and long reinforcements and cause major obstructions if spiral reinforcements are present in the pile breaking area. In order to minimize the surface tension of the pile and thus the risk of spalling on the pile, a vertical cut must also be made with a diamond saw at the finished excavation height. The last 30 to 50 centimeters of the pile can only be removed by hand due to the risk of cracks in the pile.

The Brextor process

An alternative procedure is the Brextor method, in which the force is applied horizontally (Fig. 2). The core tension of the pile head is broken with the inner cutter, the surface tension with the outer cutter. The remaining concrete edge with the reinforcement in it can then be broken away with a pile head crusher without risking damage to the pile head. Figure 3 shows an overview of the four work steps:

1. Prepare pile with milling disk
2. Milling down to level
3. Breaking out the remaining concrete with the pile head crusher and
4. Cleaning and aligning the reinforcement

In this way, the pile head is handed over to the subsequent work with a height accuracy of +/- 1 centimeter.

In addition, the concrete structure and reinforcements remain intact. The design of Brextor allows it to be used with very little or even no lateral working space, so that excavation work can be saved. The outer cutter makes the cut to the finished dismantling height automatically; the cleaning effort is reduced to cleaning work. The method is suitable for all pile types and walls: It can be used for individual bored piles, bored pile walls or as an auger. In addition, the excavated material (80% gravel with a grain size of 0-30 mm) can very often be processed directly on the construction site.

Expansion of a railway station

The client attaches great importance to the punctual execution of the work; after all, the extension work is being carried out during ongoing railway operations. “The time pressure on this construction site is great, as various milestones have to be met,” confirms Lang. Without Brextor, it would “only be possible to keep to the schedule with a lot of resources and personnel”.
The method also proved to be advantageous in view of the limited space available, as according to Lang there was not enough room for a hydraulic hammer between the nail wall and the pile head. According to the Implenia site manager, the wall, which was up to six metres high, would have had to be moved back over a length of 300 metres just to create enough space for the pile head work using a hydraulic hammer. “This would mean a massive increase in costs”.
In this way, 80 centimetres of working space could be dispensed with on the rear side of the railway line extension, which saves 1440 cubic metres of excavation (fixed dimensions) and thus not only saves costs for excavation, transport, landfill fees and material replacement, but also protects the environment because at least 280 truck journeys of 40 kilometres each were eliminated. This reduced the carbon footprint and the population benefited from a less congested road transport network.

Brextor was in operation at Liestal station (Switzerland), where the expansion of the station entrances to four lanes is expected to significantly reduce crossing conflicts and ensure more punctual and disruption-free rail traffic. “If the piles were removed by hand using hammers, there would always be a risk of damaging the reinforcements and disturbing the concrete quality at the pile head,” says Stefan Lang, site manager at Implenia, which is carrying out the removal work. “I can rule out this risk with Brextor.”